Metastable states of floating crystals

Metastable states of floating crystals

Metastable states of floating crystals

Animation of the growth of a floating crystal from N=3 to N=19 particles. The particles are added one by one to the surface of the liquid under a magnetic field. The attractive capillary forces are counterbalanced by the repulsive magnetic forces. The assembly shows a typical symmetry of atomic crystals. Credit: University of Liège / N. Vandewalle

A research team led by GRASP—Groupe de Recherche et Applications en Physique Statistique—of the University of Liège (Belgium), shows how to manipulate the mesh, shape and symmetry of floating crystals by walking, in a controlled way, between their metastable crystal states. This study is published in the journal Scientific reports.

Multi-particle systems are of interest in several fields of physics. Their structure is governed by their interactions. In particular, in the presence of attractive interactions, these systems tend to self-assemble, minimizing their energy. This phenomenon exists at all scales, governing the formation of molecules and planetary systems. Depending on the complexity of the interactions, the particles can form simple periodic structures (crystals) or more complex like protein chains.

Magnetocapillary interactions between particles allow the self-assembly of floating crystals along liquid interfaces. For a fixed number of particles, different states with different symmetric characteristics, called metastable states, coexist. Various pioneering works have observed the existence of metastable states in floating crystals.

As different states coexist, it is difficult to control the formation of specific structures. However, controlling the formation of metastable states is a key ingredient for functionalizing such assemblies, opening the way to self-assembled microrobots, for example. How to control the state of a floating crystal has never been studied before.

“Self-assembly has attracted interest from academia and industry because of its use to fabricate tiny structures,” says Nicolas Vandewalle, professor of physics and director of GRASP. “Indeed, some structures are too large to be prepared by chemical synthesis and too small to be assembled by robotic methods. In particular, the micrometer-millimeter scale is usually the bottleneck between standard bottom-up and descending.”

One of the main characteristics of self-assembled systems is that due to the high number of degrees of freedom, there are often several local minima in addition to the global minimum energy state. These metastable states can be observed at all scales, at the molecular level, in colloids, at the mesoscopic scale and at the macroscopic scale.

Metastable states of floating crystals

Different assemblies of N particles on the surface of the liquid. For each number N of particles, two different assemblies are represented face to face, demonstrating the metastability of the assembly. Credit: University of Liège /N.Vandewalle

Interest in exploiting these metastable states for active patterning has recently increased. A fundamental question, to which the researchers addressed themselves in this study, is therefore to define the conditions of navigation between the different metastable states.

“In the study that we have just published, specifies Ylona Collard, researcher at GRASP and first author of the article, we studied magnetocapillary self-assemblies composed of 3 to 19 particles. For a fixed number of particles composing the assembly, several different states coexist, distinguished by their shape, their mesh and their symmetry.

The researchers proposed two different but complementary experimental techniques to navigate in a controlled manner between these different states. The first allowed a change of state for a fixed number of particles. This is achieved by applying a horizontal magnetic field which induces a deformation of the assembly.

After relaxation, the whole will have changed state with a certain probability. The second technique controls the growth of an assembly by choosing the desired state for an assembly of N (number) + 1 beads from an assembly of N beads. An infrared laser is applied to the surface of the water to generate thermocapillary flows, controlling the trajectory of the new pearl added to the system.

“Models have been proposed to study the frequency of occurrence of the different states of an assembly when it is created, explains Nicolas Vandewalle, and to model the two experimental techniques. The simulations are in very good agreement with the experimental results. An analogy between these magnetocapillary assemblies, which can be reduced to a smaller size scale, and colloidal crystals has been proposed to broaden the perspectives of this work.”

This work is indeed relevant for the fabrication of microscopic structures such as electronic circuits, microrobots or new materials with new physical properties.

Trigger Microscale Self-Assembly Using Light and Heat

More information:
Ylona Collard et al, Controlled transitions between metastable states of 2D magnetocapillary crystals, Scientific reports (2022). DOI: 10.1038/s41598-022-20035-8

Provided by the University of Liège

Quote: Metastable States of Floating Crystals (2022, September 30) Retrieved October 1, 2022 from

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